The preferred field of application of the present invention is rectification in vacuo of materials sensitive to elevated temperatures, e.g. lactams, fatty acids, polyhydric alcohols, high-boiling ethers, various oils, food and pharmaceutical products.
In view of the speedy development in the last decades of organic production we face a problem of providing rectifying apparatuses for the separation and refining of organic materials which are either intermediate or slot products. These materials are usually sensitive to the heat effect. Inasmuch as the temperature of the processed material in the course of rectification is equal to its boiling point which latter depends on the pressure in the apparatus, a general tendency is to rectify the heat-sensitive materials in vacuo.
In order to ensure a pressure which is, if possible, equally low in all the points of the rectification apparatus, these apparatuses must offer a very low hydraulic resistance to the passage of vapours. Most frequently the preferred type of the equipment in this case are gas-liquid film-type contact apparatuses.
Known in the prior art is a gas-liquid contact apparatus for mass exchange processes in the liquid-gas system (see, for example, Author's Certificate No. 250878, Cl. 12a 5 of Feb. 2, 1968, USSR) comprising a shell with branch pipes for the admission and discharge of gas and other branch pipes for the admission and discharge of liquid, horizontal refluxed plates arranged in the shell one above another with gaps between them, and means for delivering liquid on the reflux plates. The gas admission and discharge branch pipes are arranged horizontally on the same level. Each reflux plate has outflow holes at one of its ends, and each pair of vertically adjacent reflux plates has outflow holes located at the opposite ends. On each reflux plate the liquid moves from one of its ends having no outflow holes to its other end which has outflow holes. The gas passes through the gaps between the reflux plates from one of their ends to the other, contacting with the liquid film on said plates. Within the apparatus the liquid moves from the top down over a zigzag trajectory, flowing from one reflux plate onto another through outflow holes. If the liquid film on one of the reflux plates moves in a counter-current relative to the gas then on the adjacent overlying and underlying plates the liquid film moves in a forward flow relative to the gas.
A disadvantage of the prior art apparatus consists in its limited efficiency since it fails to comply with the principle of counter-current flow of the gas and liquid. This apparatus when used for rectification fails to provide more than one theoretical plate (theoretical separating stage).
Also known in the prior art is a counter-current gas-liquid contact apparatus (see, for example, U.S. Pat. No. 3,928,513, Cl. 261/113) comprising a shell with branch pipes for admitting gas into the lower portion of the apparatus, discharging it from the upper portion of the apparatus, admitting liquid into the upper portion of the apparatus and discharging it from its lower portion, reflux plates arranged in the shell one above another with gaps, and means for delivering the liquid onto the reflux plates. Each reflux plate is horizontal and has a plurality of outflow branch pipes.
During operation of the known apparatus the gas enters it through a branch pipe in the lower portion of the shell, crosses all the reflux plates, passing through the outflow pipes and contacting the liquid film on the upper surface of the reflux plates. Then the gas is discharged outside through a branch pipe in the upper portion of the shell. The liquid is delivered onto the uppermost plate through a branch pipe in the upper portion of the shell, spreads over it and drips down through the outflow pipes onto the underlying reflux plate also spreading over its entire surface in a thin layer (film). Flowing from one reflux plate onto another and wetting these plates, the liquid arrives into the lower portion of the apparatus wherefrom it is discharged outside through a branch pipe. Within the entire apparatus there is a counter-current between the gas and liquid which ensures a large number of theoretical plates in a single apparatus.
A disadvantage of this prior art apparatus consists in the limitations with respect to the efficiency of mass exchange (e.g. during rectification), said limitations being imposed by some design features of the apparatus.
Firstly, the surface of contact cannot be developed by decreasing the gaps between the reflux plates since said gaps must be wider than the height of the outflow pipes installed on the underside of said plates. Decreasing the gaps will interfere with the liquid flow from one reflux plate onto another and reduce the throughput of the apparatus with relation to the gas phase which may lead even to flooding of the apparatus.
Secondly, the gas flow in the prior art apparatus is insufficiently agitated during its contact with the liquid film.
Thirdly, entrainment of splashes may occur in the prior art apparatus because the drops and streams of liquid flowing from the outflow pipes of the overlying reflux plate on the underlying one may be entrained by the flow of gas. In this case a portion of the liquid returns to the overlying reflux plates which cuts down the efficiency of mass exchange.
Fourthly, the gas and the liquid film covering the horizontal reflux plate move in a counter-current relative to each other. At a certain sufficiently high velocity of gas this may lead to retardation of the liquid film flow on the horizontal reflux plates and, as a result, to inferior mass exchange characteristics of the apparatus.
Fifthly, even insignificant deviations of the apparatus from the vertical position erroneously made during installation break the continuity of the liquid film on the horizontal reflux plates and the resultant incomplete wetting of said plates impairs the efficiency of mass exchange in the apparatus.